The performance of wireless mobile computing devices is affected by the capabilities of the underlying network technologies. To provide voice and data communication capabilities, cellular wireless communication systems are widely deployed, which use a variety of radio access technologies (RATs). Such systems may be multiple-access systems able to support communication with multiple users by sharing system resources such as bandwidth, frequency, and transmission power. Commonly used multiple-access systems include, but are not limited to, Long Term Evolution (LTE) systems, Code-Division Multiple Access (CDMA) systems, Time-Division Multiple Access (TDMA) systems, Frequency-Division Multiple Access (FDMA) systems, Orthogonal Frequency-Division Multiple Access (OFDMA) systems, and the like. These communication systems, sometimes standardized by organizations such as the 3rd Generation Partnership Project (3GPP) or the Institute of Electrical and Electronics Engineers (IEEE), may allow for voice communications and, additionally or alternately, may provide for the exchange of packet data, such as to, for example, access the Internet.
These communication systems may operate within licensed spectrums (e.g., frequency ranges). Licensed spectrums include portions of the radio spectrum reserved for use by licensed organizations. For example, in the United States, the Federal Communication Commission (FCC) regulates radio communications. These licensed organizations have exclusive use of the radio spectrum over geographic areas that they have been granted licenses for. One benefit of operating communication systems within licensed spectrums is the minimal interference these systems experience from other systems operating in the same frequency range. These same communication systems, however, may also operate within unlicensed spectrums. As such, communication systems operating within unlicensed spectrums may experience interference from other systems operating within the same frequency ranges. For example, LTE systems may operate within frequency ranges that another communication system, such as a Wi-Fi (e.g., IEEE 802.11n) communication system, also operates within. When two communication systems operate within the same frequency ranges, communications for one system may interfere with communications on the other system, and vice versa.
To maintain synchronization with a frequency carrier signal, communication systems may employ the use of synchronization signals. For example, in LTE systems, a base station (e.g., eNB) may transmit a primary synchronization signal (PSS) and secondary synchronization signal (SSS) at a periodic rate (e.g., once every 5 msecs) to enable user equipment (UEs) to maintain time and frequency synchronization with an LTE frequency carrier signal. UEs may also utilize transmitted cell-specific reference signals (CRSs) for similar synchronization purposes. For example, a UE operating within an LTE system may synchronize to PSS and SSS signals, and subsequently detect and acquire the CRS transmission for time and/or frequency tracking and/or automatic gain control (AGC) loop maintenance. In addition, the use of a discovery signal is proposed in LTE such that UEs are expected to detect and measure neighbor cells and/or secondary cells (Scells) using the discovery signal. The discovery signal may be, for example, a combination of PSS, SSS, and CRS subframes transmitted at a periodic rate (e.g., up to 5 msecs of populated subframes every 100 msecs). For example, LTE Release 12 UEs are expected to detect and measure at least neighbor cells and secondary serving cells using the discovery signal. FIG. 8 depicts synchronization transmissions in LTE Release 8/9/10/11 and also the discovery signal transmissions that are being considered for LTE Release 12. After an LTE UE acquires PSS/SSS it is able accurately identify LTE symbol (˜70 us) and subframe (1 ms) boundaries. Subsequently, the UE uses CRS transmissions from the cell for time/frequency tracking and AGC loop maintenance.
For communication systems operating within the Wi-Fi standard (e.g., IEEE 802.11n, infrastructure mode), access points (APs) transmit beacon frames that that contain information relevant for synchronization and discovery. For example, synchronization between APs and stations (STAs) (e.g., UE) in a Wi-Fi communication system is maintained via a timing synchronization function (TSF) where beacon frames transmitted by an AP contain a TSF timestamp value that is used by STAs to synchronize their own timer's TSF timer value. A receiving STA may accept the timing information in beacon frames sent from the AP, and if the STA's TSF timer value is different from the timestamp value in the received beacon frame, the receiving STA sets its local TSF timer value to the received timestamp value.
In addition, Wi-Fi APs may transmit beacon frames according to a beacon period (e.g., 100 msecs) that defines a series of Target Beacon Transmission Times (TBTTs) (e.g., 0 ms, 100 ms, 200 ms . . . ). At each TBTT, the AP may schedule a beacon frame as the next frame for transmission. As shown in FIG. 9, beacon frames are transmitted only if the transmission medium is determined to be free. If the transmission medium is busy (e.g., current transmissions are detected), the beacon frame transmission is deferred until the next available transmission opportunity. Subsequent beacon frames, however, are not also deferred (unless the transmission medium is busy at their scheduled TBTT), but rather are still scheduled at their pre-determined TBTTs.
In addition to the time synchronization provided by Wi-Fi's TSF mechanism described above, Wi-Fi may also include a preamble that precedes data transmission in every physical layer convergence protocol data unit (PPDU). The length of the preamble depends on a number of factors including Wi-Fi specification version (e.g., 802.1111g vs. 802.11n), the number of spatial streams, and the sounding formats supported. For example, for 802.11n, assuming mixed format preamble transmission, 4 spatial streams, non-sounding PPDU, the preamble duration is L-STF(8 us)+L-LTF(8 us)+L-SIG(4 us)+HT-SIG(4 us)+HT-STF(4 us)+4*HT-LTF(4*4=16 us)=44 us. The short training fields (STF) in the preamble can be used to determine fine timing. Preamble transmission for 802.11n is illustrated in FIG. 10. The preamble may be used by a receiving AP or UE to assist in synchronization and AGC maintenance.
Referencing back to the LTE communication system, base stations may operate primary cells (Pcell) that operate on licensed carriers (e.g., frequency carrier operating within licensed spectrum), and/or Scells that operate on licensed or unlicensed carriers (e.g., frequency carrier operating within unlicensed spectrum) (e.g., long-term evolution unlicensed spectrum (LTE-U)). If the base station operates the Scell on a licensed carrier, the base station is expected to transmit at least the following: 1) PSS/SSS with 5 ms periodicity and CRS in every subframe if the Scell does not support on/off; 2) Small cell discovery signal (SCDS—e.g. PSS/SSS+CRS, or a modified version of CSI-RS) with longer periodicity (e.g. 100 ms) if the Scell supports on/off. Additional assistance signaling regarding sequence identification information for the discovery signal and timing (e.g., periodicity and subframe offset of discovery signal) can also be provided by the Pcell. As such, the UE may use the transmitted discovery signals and assistance information to detect and synchronize to the Scell. Because the base station transmits on the Scell, which is operating on a licensed carrier, the discovery signals are transmitted with known periodicity and known symbol/subframe locations. As such, a UE can correlate with these signals and determine symbol/subframe timing for the Scell. The UE may also perform measurements on the Scell operating on a licensed carrier. For example, as is known in the art, the UE may perform radio resource management (RRM) measurements, such as measuring the Scell's reference signal received power (RSRP) and/or reference signal received quality (RSRQ). The UE, as known in the art, may also report the various measurements to a base station or some other network device.
If, however, the base station is operating an Scell on an unlicensed carrier (e.g., LTE-U), the base station is expected to implement a ‘listen before talk’ (LBT) strategy (i.e., the base station has to detect (e.g., sense) if there are transmissions on the unlicensed frequency carrier, and transmit only if no other transmission is detected on the unlicensed frequency carrier), similar to the strategy Wi-Fi employs and discussed above. Given this, it is not possible for the base station to make periodic discovery signal transmissions with a known pattern without violating the LBT strategy, as the frequency carrier may not be free for transmission during all occasions of scheduled discovery signal transmissions. As such, there are opportunities for improvements to synchronization with, and measurement reporting of, unlicensed frequency carriers.